1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of forming silicon regions on a substrate by chemical-mechanical polishing.
2. Description of Related Art
With the complication and diversification of the manufacturing process of semiconductor devices, particularly integrated circuits (ICs), polishing has been employed to remove completely a silicon layer existing on an insulating film such as a silicon dioxide film (SiO.sub.2), silicon nitride film (Si.sub.3 N.sub.4) or aluminum oxide film (Al.sub.2 O.sub.3) to thereby form a flat surface, in addition to polishing of the semiconductor substrate (wafer) itself. In this case, the polishing must be stopped when the surface of the insulating film is exposed so that only portions protruded from the surface of the insulating film are polished. To meet the requirements, it is preferable to employ selective chemical-mechanical polishing having a considerably high ratio of the rate of polishing of silicon layer to the rate of polishing of insulating film. The word "selective" means that, between the silicon and insulating material, only silicon at its protruded portions is polished. In the selective chemical-mechanical polishing, polishing proceeds in such a manner that a reaction product resulting from a reaction of silicon with a polishing liquid is removed with a polishing cloth (pad). Since the polishing liquid does not react with an insulating film, no polishing proceeds in regard to the insulating film. Thus, it is possible to obtain a high ratio of the rate of polishing of the silicon to the rate of polishing of the insulating film. The technology is described in Japanese Patent Laid-Open No. 50-99685 (1975), or in IEDM 85, pp. 688 to 691, entitled "NOVEL LSI/SOI WAFER FABRICATION USING DEVICE LAYER TRANSFER TECHNIQUE" by T. Hamaguchi et al.
On the other hand, a technology, in which an epitaxial silicon layer is selectively grown in an aperture of an insulating film formed on a silicon substrate in a mushroom fashion to form a semiconductor element therein, is disclosed in U.S. Pat. No. 3,574,008, or in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-33, NO. 11, NOVEMBER 1986, pp. 1659 to 1666, entitled "Scaled CMOS Technology Using SEG Isolation and Buried Well Process" by N. Endo et al.
However, in the case where a silicon layer is not present on the whole surface of an insulating film but partially present thereon with a certain configuration, it is impossible with the conventional chemical-mechanical polishing to polish the silicon layer uniformly so as to form a flat surface. This is because, in polishing of a silicon layer partially existing on an insulating film, polishing proceeds mainly from the corners and side surfaces of the silicon layer but it proceeds only slightly from the upper surface thereof. More particularly in the case where the silicon layer on the insulating film has a small area and/or is thick, the thickness of the silicon film is fast reduced by the effect of polishing which proceeds from the corners (upper edges) and the side surfaces, whereas, in the case where the silicon layer on the insulating film has a large area and/or is thin, polishing that proceeds from the side surfaces makes a small contribution to the reduction of the film thickness and therefore the thickness of the silicon film is not fast reduced. In other words, there is dependence of the polishing rate on the area or the thickness of the silicon layer. Accordingly, when silicon layers having various sizes are present on an insulating film, it is impossible with the conventional chemical-mechanical polishing to realize uniform polishing of all the silicon layers and formation of a flat surface.
As has been described above, in the case where a silicon layer which is partially present on an insulating film is polished by the conventional selective chemical-mechanical polishing technique, uniform polishing cannot be effected due to the dependence on the area, configuration, etc. of the single crystal (monocrystalline) silicon layer and it is therefore impossible to form a flat surface.